First problem detected... uhum..! Turns out the difference between fullscreen and windowed mode is huge. Still, fullscreen mode is what makes it look the most ugly, and the game is supposed to be made for fullscreen, so until I can get rid of the insane brightness, the problem is still there.

r_overbrightbits is on default 1, it should be 0 when testing these maps in fullscreen. That fixes it. The last thing I must find out now is if it behaves the same way in "final" compiles, because maybe the reason why it became so bright was that it was only (single test) compiles.